Introduction 1-1 Lecture 5 Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012 CS3516: These slides are generated from those made available by the authors of our text.

Application Layer 2-2 Chapter 5: outline 1.4 Performance Metrics - Continued 2.2 Web and HTTP

Network Performance Measures  Latency  usually implies the minimum possible delay. Latency assumes no queuing and no contention encountered along the path.  Goodput  {measured at the receiver} the rate in bits per second of useful traffic received. Goodput excludes duplicate packets and packets dropped along the path.  Fairness  either Jain’s fairness or max-min fairness are used to measure fair treatment among competing flows.  Quality of Service (QoS)  a QoS measure accounts for importance of specific metric to one type of application [e.g. jitter and playable frame rate for streaming media].

Wireless Performance Metrics WLANs and WSNs are concerned with packet loss and employ additional metrics: Delivery ratio  the ratio of packets received to packets sent {excluding duplicates and retransmissions}. Packet loss rate  the percentage of packets lost or dropped. Link layer retransmission rates  the percentage of DL layer frames that are retransmitted.

Packet Loss  queue (aka buffer) preceding link in buffer has finite capacity.  packet arriving to full queue is dropped (aka lost) [FIFO Drop Tail router].  lost packet may be retransmitted by previous node, by source end system, or not at all. A B packet being transmitted packet arriving to full buffer is dropped. buffer (waiting area)

Performance Metrics Summary  The three most general performance measures are: utilization, throughput and response time.  In computer networks, end-to-end delay is an important performance metric.  Queuing models are used to analyze and estimate computer network performance.

 Other useful metrics include: latency, goodput, fairness and QoS metrics such as jitter or playable frame rate.  In wireless networks, delivery ratio, packet loss rate and link layer retransmission rates are valuable network measures. Performance Metrics Summary

Application Layer 2-8 Chapter 5: outline 1.4 Performance Metrics 2.2 Web and HTTP

Application Layer 2-9 Web and HTTP First, a review…  web page consists of objects Abraham Lincoln's Gettysburg Address Abraham Lincoln's Gettysburg Address Four score and seven years ago… Now we are engaged in a great … But, in a larger sense, …font> Your webserver wrapped this text in a header so the browser would know what was coming at it.

Application Layer 2-10 HTTP overview HTTP: hypertext transfer protocol  Web’s application layer protocol  client/server model  client: browser that requests, receives, (using HTTP protocol) and “displays” Web objects  server: Web server sends (using HTTP protocol) objects in response to requests PC running Firefox browser server running Apache Web server iphone running Safari browser HTTP request HTTP response HTTP request HTTP response

Application Layer 2-11 HTTP overview (continued) uses TCP:  client initiates TCP connection (creates socket) to server, port 80  server accepts TCP connection from client  HTTP messages (application-layer protocol messages) exchanged between browser (HTTP client) and Web server (HTTP server)  TCP connection closed HTTP is “stateless”  server maintains no information about past client requests protocols that maintain “state” are complex!  past history (state) must be maintained  if server/client crashes, their views of “state” may be inconsistent, must be reconciled aside WHY PORT 80???

Application Layer 2-12 HTTP connections non-persistent HTTP  at most one object sent over TCP connection  connection then closed  downloading multiple objects required multiple connections  Connect(), send(), rcv(), close() sequence. persistent HTTP  multiple objects can be sent over single TCP connection between client, server

Application Layer 2-13 Non-persistent HTTP: response time RTT (definition): time for a small packet to travel from client to server and back HTTP response time:  one RTT to initiate TCP connection  one RTT for HTTP request and first few bytes of HTTP response to return  file transmission time  non-persistent HTTP response time = 2RTT+ file transmission time time to transmit file initiate TCP connection RTT request file RTT file received time

Application Layer 2-14 Persistent HTTP non-persistent HTTP issues:  requires 2 RTTs per object  OS overhead for each TCP connection  browsers often open parallel TCP connections to fetch referenced objects persistent HTTP:  server leaves connection open after sending response  subsequent HTTP messages between same client/server sent over open connection  client sends requests as soon as it encounters a referenced object  as little as one RTT for all the referenced objects

Application Layer 2-15 HTTP request message  two types of HTTP messages: request, response  HTTP request message:  ASCII (human-readable format) request line (GET, POST, HEAD commands ) header lines carriage return, line feed at start of line indicates end of header lines GET /index.html HTTP/1.1\r\n Host: www-net.cs.umass.edu\r\n User-Agent: Firefox/3.6.10\r\n Accept: text/html,application/xhtml+xml\r\n Accept-Language: en-us,en;q=0.5\r\n Accept-Encoding: gzip,deflate\r\n Accept-Charset: ISO ,utf-8;q=0.7\r\n Keep-Alive: 115\r\n Connection: keep-alive\r\n \r\n carriage return character line-feed character

Application Layer 2-16 HTTP request message: general format request line header lines body method sp cr lf version URL cr lf value header field name cr lf value header field name ~ ~ ~ ~ cr lf entity body ~ ~ ~ ~

Application Layer 2-17 Uploading form input POST method:  web page often includes form input  input is uploaded to server in entity body URL method:  uses GET method  input is uploaded in URL field of request line:

Application Layer 2-18 Method types HTTP/1.0:  GET  POST  HEAD  asks server to leave requested object out of response HTTP/1.1:  GET, POST, HEAD  PUT  uploads file in entity body to path specified in URL field  DELETE  deletes file specified in the URL field

Application Layer 2-19 HTTP response message status line (protocol status code status phrase) header lines data, e.g., requested HTML file HTTP/ OK\r\n Date: Sun, 26 Sep :09:20 GMT\r\n Server: Apache/ (CentOS)\r\n Last-Modified: Tue, 30 Oct :00:02 GMT\r\n ETag: "17dc6-a5c-bf716880"\r\n Accept-Ranges: bytes\r\n Content-Length: 2652\r\n Keep-Alive: timeout=10, max=100\r\n Connection: Keep-Alive\r\n Content-Type: text/html; charset=ISO \r\n \r\n data data data data data...

Application Layer 2-20 HTTP response status codes 200 OK  request succeeded, requested object later in this msg 301 Moved Permanently  requested object moved, new location specified later in this msg (Location:) 400 Bad Request  request msg not understood by server 404 Not Found  requested document not found on this server 505 HTTP Version Not Supported  status code appears in 1st line in server-to- client response message.  some sample codes :

Application Layer 2-21 Trying out HTTP (client side) for yourself 1. Telnet to your favorite Web server: opens TCP connection to port 80 (default HTTP server port) at cis.poly.edu. anything typed in sent to port 80 at cis.poly.edu telnet type in a GET HTTP request: GET /~jb/ HTTP/1.1 Host: cs.wpi.edu by typing this in (hit carriage return twice), you send this minimal (but complete) GET request to HTTP server 3. look at response message sent by HTTP server! (or use Wireshark to look at captured HTTP request/response)

Application Layer 2-22 User-server state: cookies many Web sites use cookies four components: 1) cookie header line of HTTP response message 2) cookie header line in next HTTP request message 3) cookie file kept on user’s host, managed by user’s browser 4) back-end database at Web site example:  Susan always access Internet from PC  visits specific e-commerce site for first time  when initial HTTP requests arrives at site, site creates:  unique ID  entry in backend database for ID

Application Layer 2-23 Cookies: keeping “state” (cont.) client server usual http response msg cookie file one week later: usual http request msg cookie: 1678 cookie- specific action access ebay 8734 usual http request msg Amazon server creates ID 1678 for user create entry usual http response set-cookie: 1678 ebay 8734 amazon 1678 usual http request msg cookie: 1678 cookie- specific action access ebay 8734 amazon 1678 backend database

Application Layer 2-24 Cookies (continued) what cookies can be used for:  authorization  shopping carts  recommendations  user session state (Web ) cookies and privacy:  cookies permit sites to learn a lot about you  you may supply name and to sites aside how to keep “state”:  protocol endpoints: maintain state at sender/receiver over multiple transactions  cookies: http messages carry state

Application Layer 2-25 Web caches (proxy server)  user sets browser: Web accesses via cache  browser sends all HTTP requests to cache  object in cache: cache returns object  else cache requests object from origin server, then returns object to client goal: satisfy client request without involving origin server client proxy server client HTTP request HTTP response HTTP request origin server origin server HTTP response

Application Layer 2-26 More about Web caching  cache acts as both client and server  server for original requesting client  client to origin server  typically cache is installed by ISP (university, company, residential ISP) why Web caching?  reduce response time for client request  reduce traffic on an institution’s access link  Internet dense with caches: enables “poor” content providers to effectively deliver content (so too does P2P file sharing)

Application Layer 2-27 Caching example: origin servers public Internet institutional network 1 Gbps LAN 1.54 Mbps access link assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 15%  access link utilization = 99%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs problem!

Application Layer 2-28 assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 15%  access link utilization = 99%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs Caching example: fatter access link origin servers 1.54 Mbps access link 154 Mbps msecs Cost: increased access link speed (not cheap!) 9.9% public Internet institutional network 1 Gbps LAN

institutional network 1 Gbps LAN Application Layer 2-29 Caching example: install local cache origin servers 1.54 Mbps access link local web cache assumptions:  avg object size: 100K bits  avg request rate from browsers to origin servers:15/sec  avg data rate to browsers: 1.50 Mbps  RTT from institutional router to any origin server: 2 sec  access link rate: 1.54 Mbps consequences:  LAN utilization: 15%  access link utilization = 100%  total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs ? ? How to compute link utilization, delay? Cost: web cache (cheap!) public Internet

Application Layer 2-30 Caching example: install local cache Calculating access link utilization, delay with cache:  suppose cache hit rate is 0.4  40% requests satisfied at cache, 60% requests satisfied at origin origin servers 1.54 Mbps access link  access link utilization:  60% of requests use access link  data rate to browsers over access link = 0.6*1.50 Mbps =.9 Mbps  utilization = 0.9/1.54 =.58  total delay  = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache)  = 0.6 (2.01) (~msecs)  = ~ 1.2 secs  less than with 154 Mbps link (and cheaper too!) public Internet institutional network 1 Gbps LAN local web cache

The End is Near!